Many optoelectronic sensors generate a received signal from a light spot on their light receiver and evaluate it. As a rule, it is a light spot of an associated light transmitter. In some sensors such as through-beam sensors, the light transmitter is installed at a distance. With reflection light barriers or sensors according to the scanning principle, the light transmitter is located at the light receiver. A light beam is transmitted into the monitored zone and the light beam reflected by objects or by a separately set up reflector is received again. The distance can also be measured by a determination of the time of flight in this respect. A distance measurement is also possible by triangulation, wherein the light transmitter and the light receiver are arranged offset at a base distance such that the light spot migrates over the light receiver in dependence on the distance. The previously named one-dimensional sensors can be extended to include surface scanning or spatial scanning, for example by a multiple arrangement in a light grid or by moving the light beam such as in a light scanner.
A number of optical sensors are exposed to large reception dynamics, i.e. the intensity of the light spot fluctuates considerably within the measurement scene. In this respect, measurement distances and remissions of the respective remitting object that vary in particular play a role. A plurality of decades lie between a distant, dark object and a near, reflective or shiny object with a corresponding angle of incidence. Sensors should cover a dynamic range that is as large as possible for a reliable measurement. This region is practically restricted by the noise at the lower end and is restricted by overmodulation effects or saturation at the upper end.
A number of conventional measures are known to expand the dynamic range. One variable is the gain of the light receiver by which the sensitivity is changed. However the speed or flank steepness of the received signal is simultaneously adjusted with this and this has unwanted effects on the measured distance with many time of flight methods. A further measure also with a respectively given sensitivity of the light receiver is the setting of the exposure time or of the point in time of the detection (gating). This is likewise problematic in time of flight processes since a speed change of the received signal can hereby result. In addition, particularly with highly integrated light receivers, the named parameters are often not accessible at all and the reconfiguring of the sensor parameters requires time.
It is furthermore conceivable to use mechanical shutters to adapt the power incident on the light receiver. This avoids said problems. The mechanical shutters, for instance iris shutters, are however, very sluggish so that the required adaptation of the shutter diameter brings along a latency time that is greater than the measurement rate of the sensor that is aimed for. In addition, the aimed-for shutter diameter is frequently not set with the required precision.
A situation-based adaptation of the light intensity is also possible at the transmission side. The power of the light transmitter is changed for this purpose. With pulse processes, the amplitude of the pulses is not necessarily varied for this purpose, but rather the pulse length or the pulse/pause ratio. With time of flight sensors, an adaptation of the transmission power or of the pulse behavior can in turn cause a change in the flank steepness and can change the measurement result. A change of the pulse/pause ratio is not even possible without difficulty in dependence on the measurement method since this ratio carries information for the time of flight measurement under certain circumstances. In addition, with pulsed operating modes having very high repetition frequencies, it can occur that the required reconfiguration brings about a latency time that is too slow for the aimed-form measurement rate of the sensor. Specific performance levels are standardized in EN 13849 and safety integrity levels (SILs) are standardized in EN 61508 for sensors that are used in safety engineering for monitoring a hazard source and that bring it into a safe state on recognition of a hazard. The observation of said levels during reconfiguration too can require a check with time-consuming test routines.
A transmission optics or reception optics having optical elements such as lenses is provided in practically every optical sensor. This optics is frequently focused to a specific distance or distance range with the aid of a focal adjustment in that the position of the lenses and thus the back focal length of the transmission optics or reception optics is adjusted electromechanically or optomechanically. Alternatively, liquid lenses are also known that directly change the focal length for a focal adjustment.
In a further development of liquid lenses for focal adjustment, EP 2 071 367 A1 proposes also varying the tilt of the liquid lens by applying different voltages in the peripheral direction. In a camera, this serves to prevent a recording of blurred images in that the camera's own movement is determined and the liquid lens is tilted to counteract this movement.
A further optoelectronic sensor having a liquid lens is disclosed in DE 10 2005 015 500 A1 whose beam shaping properties is asymmetrically variable by an asymmetrical frame or by different electrical potentials at separate electrodes of the lens frame. However, the document does not then explain the purpose for which this can be used.